Work machines for moving mass quantities of earth, rock, minerals and other material commonly comprise a work implement configured for loading, such as a bucket controllably actuated by hydraulic cylinders. An operator manipulates the work implement to perform a sequence of distinct functions using one or more control handles to generate operator command signals. In a typical work cycle for loading a bucket, the operator first maneuvers the work machine close to a pile of material and levels the bucket near the ground surface, then directs the machine forward to engage the pile.
The operator subsequently raises the bucket through the pile, "racking" (tilting back) the bucket in order to capture the material. When the bucket is filled or breaks free of the pile, the operator fully racks the bucket and lifts it to a dumping height, backing away from the pile to travel to a specified dump location. After dumping the load, the work machine is returned to the pile to begin another work cycle.
It is increasingly desirable to automate the work cycle in order to reduce operator fatigue. Systems are known which provide teaching and playback functions to perform repetitive digging operations using a hydraulic excavator, such as are disclosed in U.S. Pat. Nos. 5,274,557 and 5,359,517 to Moriya et al. Stored command signals forming a complete dig-transport-dump-return work cycle are repeatedly played back in a closed-loop sequence. There has heretofore been no corresponding functionality provided for tracked or wheeled loaders, in part because operator judgment remains useful for selection of pile entry points and maneuvering the work machine. If playback of a learned loading sequence is begun too soon or too late, the machine may stall or fail to fill the bucket.
At present, operator assistance for a wheel loader typically takes the form of a system for computing implement command signals based upon sensed machine parameters. For example, U.S. Pat. No. 3,782,572 to Gautler discloses a hydraulic control system which monitors wheel torque to automatically increase lift cylinder pressure when wheel slip is detected. U.S. Pat. No. 5,528,843 to Rocke discloses a control system for capturing material which selectively supplies maximum lift and tilt signals in response to sensed hydraulic pressures. International Application No. WO 95/33896 to Dasys et al. discloses sensor feedback for automatic bucket loading, in which the direction of fluid flow to the hydraulic cylinder is automatically reversed when bucket forces exceed allowable limits.
While the foregoing systems advantageously may optimize loading efficiency, depending upon the sophistication of the control algorithm, an operator may prefer to substitute his own judgment and experience to define a "standard" sequence of implement command signals for a wheel loader.
The present invention is directed to overcoming one or more of the problems as set forth above.